There's a marginal difference between 2.8Ghz Core2 Duo and a 2.66Ghz Core2 Duo. You'll probably be able to get HD streaming, but don't expect to get the highest of resolutions.
The range of Slingbox's HD resolutions are listed below.
- 1280 x 720
- 1440 x 540
- 1680 x 540
- 1920 x 540
Honestly, I think the more important numbers to determine HD capability is the bandwidth between your home and your in-laws. You'll need to be able to achieve a sustain speed of 3 Mbps or higher between the connection. This means your in-laws upload speed needs to be at least 3Mbps and your download speed needs to be atleast 3Mbps. Just be aware that most ISPs advertise their speeds as "burst" as opposed "sustained".